1. Field of the Invention
The present invention relates to an image heating apparatus such as a fixing device used in a copying machine, a printer and the like.
1. Related Background Art
In conventional electrophotographic image forming apparatuses, a toner image visualized on a recording sheet is generally fixed to the recording sheet by means of a heat roller a temperature of which is controlled and a pressure roller having an elastic layer therearound and urged against the heat roller in such a manner that the recording sheet is subjected to heat and pressure while being conveyed between the heat roller and the pressure roller.
However, recently, in order to save electric power and to decrease a time from the power ON to the image output, as disclosed in the Japanese Patent Application Laid-open Nos. 63-313182-and 2-157878, there has been proposed a fixing apparatus (of film-heating fixing type) comprising a heater unit including a fixedly supported heating body (heater) and a heat-resistive film (fixing film) conveyed while being urged against the heater, and a pressurizing member for closely contacting a recording sheet against the heater unit. Wherein a toner image formed on the recording sheet is fixed to the recording sheet by applying heat from the heater to the recording sheet through the film.
In FIG. 19 showing an example of such a film-heating fixing apparatus, the apparatus comprises a heater unit 60 including a cylindrical heat-resistive film 65 which is constituted by a base layer formed from a polyimide film having a thickness of 40 to 60 .mu.m and, a mold releasing layer made of PFA including PTFE dispersed therein and having a thickness of 5 to 10 .mu.m and providing an outer peripheral surface (contacting with a recording sheet and a toner image). A heater (heating body) 61 is constituted by an insulation ceramic base plate having low heat capacity and also having a longitudinal direction perpendicular to a conveying direction of the recording sheet 20, a heating resistor 62 printed on a surface of the base plate along the longitudinal direction thereof, and a temperature detecting elements (such as a thermistor) 63 contacted with a surface of the base plate opposite to the heating resistor. The heater 61 is heat-insulated and fixedly supported by a film guide (heater stay) 66 having a semi-circular U-shaped cross-section in such a manner that the heating resistor 62 is exposed outside. Further, temperature control of the heater 61 is effected by controlling electric power supplied from a power source 35 to the heating resistor 62 by driving a Triac 55 by means of a CPU 101 in response to output of the temperature detecting element 63.
An inverted U-shaped reinforcing plate member 67 serves to prevent deformation of the heater unit 60 (including the heater 61, thermistor 63, heater stay 66 and the like) when the heater unit is pressurized by a pressure roller 7. Incidentally, an inner diameter of the fixing film 65 is selected to become greater than an outer peripheral length of the heater unit including the reinforcing plate member 67. The pressure roller 7 is urged against the heater 61 by means of a pressurizing means (not shown) with total pressure of 9 to 11 Kgf. Further, the pressure roller 7 is rotated in the conveying direction (anti-clockwise direction) of the recording sheet 20 by means of a drive system (not shown). As a result, the cylindrical fixing film 65 is rotated around the film guide 66 while slidingly contacting with the heating resistor of the heater 61. In this case, in order to reduce sliding friction between the heater and an inner peripheral surface of the film, heat-resistive grease is disposed between the heater and the film.
With this arrangement, when the recording sheet 20 is guided between the film 65 and the pressure roller 7, since the recording sheet passes through a fixing nip, the toner image formed on the recording sheet is fixed to the recording sheet. In the film-heating fixing apparatus, since the heat capacity of the heater can be reduced to 1/10 of those of conventional fixing devices of heat roller type and any heating body having higher temperature increasing ability can be used, a time period for permitting the heater to fix the toner image to the recording sheet can be reduced to several seconds. Thus, unlike to the conventional fixing devices of heat roller type, so-called on demand fixing can be realized.
Since the film-heating fixing has low heat capacity, temperature ripple is increased in ON/OFF control which is commonly used in the roller type fixing devices. In consideration of this fact, accuracy of the temperature control is improved by changing electric power in accordance with a difference between a target temperature and an actual temperature by using a means for continuously or steppingly changing the electric power by phase control or wave number control. However, the following problems occur.
(Harmonic Wave Distortion)
When AC voltage is applied to a non-linear circuit having a switching element(s), harmonic wave current is generated. When commercial voltage of 50, 60 Hz is switched by the phase control, harmonic wave current is generated due to non-linearity of the circuit. In general, transformers of the commercial power source (normally arranged on electric poles) can transform frequency voltage of 50, 60 Hz efficiently (without heat loss); however, regarding the harmonic wave current, the transforming efficiency is worsened, thereby generating heat. In consideration of this fact, in power plants, since not only electric power required for the apparatus but also electric power corresponding to heat loss in the transformer must supply, energy efficient is worsened. The harmonic wave distortion in the wave number control is less than that in the phase control. The reason is that, in the wave number control, zero-cross control for effecting ON/OFF when the power source voltage reaches to zero or therearound is effected, whereas, in the phase control, ON/OFF is effected at considerably higher voltage.
(Flicker)
Since the heat capacity is small in the on demand fixing, the accuracy of the temperature control is improved by changing the electric power frequently. By doing so, the electric power is fluctuated frequently more than in the conventional roller type fixing devices (for example, in the roller type fixing devices, since the heat capacity is great, the temperature can be kept constant by merely changing the electric power every five seconds; however, in the on demand fixing, the temperature is not kept constant so long as the electric power is changed by several times within a second). The change in the consumption electric power (consumption current) causes the change in the power source voltage. In particular, when a power source having high line impedance is used (for example, when a power source is installed remote from a transformer disposed on an electric pole and resistance of a transmission line between the power source and the transformer is great), the power source voltage is changed frequently and greatly. As a result, illumination and/or TV screens sometimes flicker (such a phenomenon is referred to as "flicker" hereinafter). The flicker due to the wave number control is greater than the flicker due to the phase control. The reason is that, in the phase control, since the frequency of change in current is greater than 100 Hz, the flicker cannot be ascertained by human's eyes; whereas, in the wave number control, when the electric power is controlled by ten steps, for example, by dividing every ten half waves into a respective group and by turning ON several half waves in the former half and turning OFF several half waves in the latter half, since the frequency of change in current becomes about 10 Hz, the flicker can easily be ascertained by the human's eyes.
As the electric power of the heater is increased, the harmonic wave distortion and/or the flicker are worsened, because, upon power ON, the change in current is increased.